It has been said by top industry experts that slip-stick is the single greatest problem for modem oil and gas well drilling. Other industry technical experts have said that axial bit vibrations and/or bit bounce comprise the most significant problem in oil and gas well drilling. According to studies of these problems made by the inventors, which studies comprise insights into these problems that are part of the present invention, it has been concluded and demonstrated in computer simulations, as discussed hereinafter, that the two problems are closely related and, in fact, are both directly synonymous with drill string torsional vibrations or oscillations.
Whenever the drill bit is rotated for drilling into a formation, the drill string has torsional windup or torsional potential energy, just as a torsional spring might have when torque is applied thereto. When drilling, it is highly desirable that this torsional windup or potential energy be a constant value based on the torsional constant of the drill string, and not a varying or oscillating amount. The drill pipe diameter and well depth are significant factors in determining the drill string torsional spring constant.
The windup that occurs is basically stored elastic potential energy. The drill string torsional energy may be altered by bit weight, bore hole friction or cutting conditions whereby more or less windup is induced into the drill string. The drill bit speed is reduced proportionally by an increase in torque. If the torque increases enough, the drill bit stops rotation completely. However, since rotational power is still being applied to the drill string for drilling, the drill string continues to windup (increasing elastic potential energy). When the windup (stored elastic potential energy) is great enough to overcome the increase in torque which stopped the bit, the stored up potential energy becomes kinetic energy which accelerates the drill string, BHA and the drill bit. The drill string, BHA and drill bit accelerate rapidly and will accelerate faster than, for instance the top drive input rpm, due to the stored elastic potential energy that is now much more than is required to turn the drill string, BHA and drill bit at the original torque (RPM).
The bit, BHA and drill string speed (RPM) increases until it rotates faster than the input speed (RPM) from the original drive causing the drill string to unwind more than required. The excessive unwinding releases more stored elastic potential energy than what is required to drive the drill bit at the original torque (RPM) and starts harmonic motions, such as but not limited to axial movements (bit bounce) and Slip-Stick (Stick-Slip).
The windup and unwinding causes the entire drill string to shorten and then lengthen. The speed changes from near zero rpm or zero rpm to speeds greater than the drill string drive constant input speed, thereby inducing full-blown slip-stick (stick-slip) and bit bounce. In the past, the cycles torsional oscillations continue until the driller removes WOB or there are connection failures.
Drill string torsional vibrations occur frequently during drilling. In very general terms, torsional stress is caused when one end of the drill string is twisted while the other end is held fixed or is twisted in the opposite direction. The long length of the drill string will normally store a significant amount of torsional energy when drilling. When torsional vibrations become severe, they can escalate into slip-stick oscillations whereby the bit may briefly stop turning or at least slow down until sufficient torque is developed at the bit to overcome static friction. When the stalled bit breaks free, it may do so at rotational speeds from to two to ten times the surface rotational speed. For example, when drilling at 200 rpm, slip-stick variations may produce drill bit rotational rpm variations between zero and 2000 rpm.
As discussed above, the accompanying twisting and untwisting of the drill string produces changes in the axial length of the drill string. Because modern PDC cutting elements of bits have a very short length and, ideally, must be held in constant close contact with the surface to be cut for maximum cutting effects, even small axial changes in the length of the drill string can significantly impede drilling progress and can cause bit bounce.
Moreover, torsional slip-stick is often regarded as one of the most damaging moues of vibration. The fluctuating torques in the drill-string are difficult to control without repeatedly pausing drilling. Torsional slip-stick almost invariably causes damage to the bit or drill-string. Even small amplitude slip-stick vibrations are thought to be a major cause of bit wear.
Torsional vibrations can be set off by torque fluctuations which may occur through changes in torque applied to or by the drill string which may arise for many reasons. As non-limiting examples, changes in torque may occur due to changes in the lithology, frictional forces along the well bore, changes in bit weight and/or stabilizers sticking in soft formations. It will be understood that large amounts of torsional energy will be stored in the drill string in response to applying the necessary torque for rotating the drill bit to cut through the formation. Torsional vibrations also affect the borehole and may produce a twisted borehole that becomes the source for additional torque. Thus, the problem of torsional vibrations is self-reinforcing. For many reasons, it is desirable to drill a straighter hole with reduced spiraling effects along the desired drilling path and with fewer washed out sections. For instance, it has been found that tortuosity, or spiraling effects frequently produced in the wellbore during drilling, are associated with degraded bit performance, bit whirl, an increased number of drill string trips, decreased reliability of MWD (measurement while drilling) and LWD (logging while drilling) due to the vibrations generally associated therewith, increased likelihood of losing equipment in the hole, increased circulation and mud problems due to the troughs along the spiraled wellbore, increased stabilizer wear, decreased control of the direction of drilling, degraded logging tool response due to hole variations including washouts and invasion, decreased cementing reliability due to the presence of one or more elongated troughs, clearance problems for gravel packing screens, decreased ROP (rate or speed of drilling penetration), and many other problems.
When drilling wells, it is highly desirable to drill the well as quickly as possible to limit the costs. It has been estimated that doubling the present day rate of drilling would result in cost savings to the oil industry of from two hundred to six hundred million dollars per year. This estimate may be conservative.
During the drilling of a well, considerable time is lost due to the need to trip the drill string. The drill string is removed from the wellbore for any of various reasons, e.g., to replace the drill bit. Reducing the number of drill string trips, especially in deep wells where removal and replacement of the drilling string takes considerable time, would greatly reduce drilling rig daily rental costs.
While the design of drill bits has often been the chief focus in the prior art to reduce many of the problems discussed above, some efforts have been made to improve other aspects of the bottom hole assembly. The typical bottom hole assembly includes a plurality of heavy weight drill collars. The typical steel heavy weight collars are relatively inexpensive and durable. However, due to their size and construction, prior art weight collars are unbalanced to some degree and tend to introduce variations. Moreover, even if they were perfectly balanced, the heavy weight collars have a buckling point and tend to bend up to this point during the drilling process. The result of imbalanced heavy weight collars and the bending of the overall downhole assembly produces a flywheel effect with an imbalance therein that may easily cause the drill bit to whirl, vibrate, and/or lose contact with the wellbore face in the desired drilling direction.
Efforts have also been made to make heavier drilling collars. For instance, it has been attempted to increase the diameter of steel drill collars to provide increased weight adjacent the drill bit. However, this then decreases the annular space between the higher diameter steel drill collars and the wall of the bore hole. The decrease in annular space creates a significant washout of the hole due to the necessarily higher velocity mud flow through a smaller annulus, especially in uncompacted formations. The inventors have provided improved drilling collars which result in many benefits as per U.S. Patent Application No. 60/442,737, which is incorporated herein by reference. However, even with a significant increase in weight directly above the bit as taught by the inventors therein, the effects of slip-stick are reduced but may not be stopped altogether as can be demonstrated by the computer program simulation developed by the inventors and discussed herein. Examples of utilizing the improved drilling collars as compared to standard drilling collars under conditions which may cause slip stick are provided hereinafter.
An article from Offshore Magazine, issued August 2001, written by Chen et al., entitled “Wellbore design: How long bits improve wellbore micro-tortuosity in ERD operations,” discloses tortuosity as one of the critical factors in extended reach well operations, having two components: macro- and micro-tortuosity. The effects include high torque and drag, poor hole cleaning, drill string buckling, and loss of available drilled depth, among other negative conditions. A new drilling system using long gauge bits significantly reduces hole spiraling, one form of micro-tortuosity, which is intended by use of the drill bit design to improve many facets of the drilling operation.
The above cited prior art does not provide a reliable means for preventing slip-stick during drilling. Consequently, there remains a need to provide an improved downhole assembly to perform this function. Those of skill in the art will appreciate the present invention which addresses the above problems and other significant problems.